Limitations on the LCD VXD Inner Radius due to Backgrounds

1
Limitations on the LCD VXD Inner Radius due to
Backgrounds

• Tom Markiewicz/SLAC
• SLAC LCD Meeting
• September 5, 2000

2
The Experts

• Takashi Maruyama (SLAC)
• Pairs and Neutron Backgrounds
• Jeff Gronberg (LLNL)
• Pair and Neutron Backgrounds
• Stan Hertzbach (U. Mass)
• Synchrotron Radiation

3
Background Sources Important to the VXD

• Charged Particles
• Direct Hits from ee- pairs produced in the
  beam-beam interaction
• Charged secondaries made when beam-beam ee-
  pairs hit something else
• Neutrons
• Beam Dump
• Lost Particles
• Pairs
• Radiative Bhabhas
• Disrupted Beam
• Photons
• Synchrotron radiation from final focus,
  especially the final doublet
• Photons made when beam-beam ee- pairs hit
  something else

4
Caveats and Excuses

• Almost all quantitative work done for
• ZDR final focus
• L 2
• Small Detector
• 1 TeV c.o.m.
• Current Model
• Raimondi Final Focus
• Integrate local chromaticity correction into
  final quad doublet by interleaving sextupoles
• L 4.3 m
• Chosen because it would place QD0 (first quad)
  OUTSIDE the (closed) door of the Small Detector
• Large Detector
• Once charged secondaries from pairs splattering
  on QD0 are absorbed by low Z mask, dont need 6
  Tesla to control VXD hit density
• 500 GeV c.o.m.
• Politics

5
Introduction to the Beam-Beam InteractionSR
photons from individual particles in one bunch
when in the electric field of the opposing bunch

• Pinch makes beamstrahlung photons
• 1.5E10 per bunch _at_ ltEgt30.3 GeV (0.83 Mw)
• Particles that lose a photon are off-energy
• Physics problem luminosity spectrum
• Extraction line problem
• 1 TeV design has 77 kW of beam with lt 50 E_nom,
  4kW lost (0.25 loss)
• Working plan Ignore for now- not a problem _at_ 500
  GeV _at_ 1 TeV either measure Pol, E upstream,
  steal undisrupted pulses for diagnostics,
  calibrate other
• Photons themselves go straight to dump
• Not a background problem, but angular dist. (1
  mrad) limits extraction line length
• Photons interact with opposing e,g to produce
  e,e- pairs and hadrons

gg ? ee- (Breit-Wheeler) eg ? eee-
(Bethe-Heitler) ee ?eeee- (Landau-Lifshitz)
gg ? hadrons
6
e,e- pairs from beams. gg interactions44K per
bunch per side _at_ ltEgt10.5 GeV (0.85 W)
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Direct Pairs

• Pt of ee- from given bunch Sum of
• Pt from individual pair creation process
• small
• Pt from collective field of opposing bunch
• large
• limited by finite size of the bunch

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Dead-Cone Formalism
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ee- Pair pT vs. theta Distribution
Hard edge from finite beam size
High pT inside cone
e,e- with high intrinsic pt can hit small radius
VXD
Low pt/high angle curl in field
50 mrad
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Pair Stay-Clear from Guinea-Pig Generator and
Geant
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Another View of the Pairs
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Pair Stay-Clear from Analytic Formalism(NLC-1000A
Beam Parameters)
VXD-LCD-L2
3T
4T
6T
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e,g,n secondaries made when pairs hit high Z
surface of LUM or Q1
High momentum pairs mostly in exit beampipe
Low momentum pairs trapped by detector solenoid
field
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Charged Secondaries

• Spiral in detector field back to VXD
• To remove
• Shield the field lines which direct secondaries
  back to VXD layer with low Z absorber
• For L 2m case
• 10cm long x 2mm thick Be RING Mask at z 65cm
• inside vacuum pipe
• 15cm behind tip of M1 so secondaries that it
  itself produces are shielded
• becomes the limiting aperture for SR radiation
  the further away from the IP it is, the worse a
  limit it becomes
• perhaps fine tuning solenoid fringe field will
  help

15
New Masking
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1.2 cm VXD L1 in BOTH L S Detectors
Black Layer 1 Turquoise Layer 2 Green Layer
3 Blue Layer 4 Red Layer 5
3.5 x more Layer 1hits at 3 Tesla
Hits / bunch
With few backscattered hits, LCD group currently
feels aggressive 1.2 cm VXD is also possible for
Large Detector (3-4 T) detector
B (Tesla)
2.0 hits/mm2/train 84 from multiple hits by
primary pair electrons
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New Large Detector model

• Update the Large detector model
• Change VXD to same design as the small detector.
• Assume 4T B-field
• Move inner edge of M1 out to respect the pair
  edge.
• Add a second ring to shield SVX layer 2

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SVX Backgrounds for new LCDs
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Masking Charged Secondaries When L 4.3m
For LCD-L -1.2cm field line falls on Q1, where it
is easy to put shielding, other lines fall in
apertures. We need to study if this is a problem,
but for now assume that a low Z mask position can
be found that does not interfere with SR or beam
and is far from the IP
For LCD-S 1.2cm field line is outside all
relevant apertures by z 4.3m
20
Total VXD Neutron Backgrounds

• Neutron Sources
• e/e- pairs and radiative bhabhas hitting
  beam-pipe, luminosity monitor, masks and magnets
  in the extraction line 1cm radius aperture
  beginning at 6 m
• Disrupted beam lost in the extraction line.
• 0.25 beam loss in recent redesign
• Beam (10 MW _at_ 1 TeV) and beamstrahlung photons
  (1 MW _at_ 1 TeV) in the dump

Neutron hit density in VXD _at_ 1.2 cm (2m L
Design, 1 TeV c.o.m. ) Beam-Beam pairs (small
det.) 1.9 x 109 hits/cm2/yr Beam-Beam pairs
(large det.) 4.4 x 109 hits/cm2/yr Radiative
Bhabhas 0.01 x 109 hits/cm2/yr Beam loss in
extraction line 0.01 x 109 hits/cm2/year Backshine
from dump 0.25 x 109 hits/cm2/yr TOTAL 2.2-4.7
x 109 hits/cm2/yr
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Neutrons produced by Pairs and Rad. Bhabhas
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Neutrons produced by Pairs and Rad. Bhabhas that
hit the VXD

• Neutrons which reach the IP are produced close to
  the IP, mainly in the luminosity monitor
• At z0, the flux of these neutrons is independent
  of r. While detailed simulations are needed, we
  anticipate rate will go down by a factor of 4 as
  L doubles.

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Extraction Line Beam Loss Not a VXD Problem150
m long with common g and e- dump
Problem Handling the large low E tail on the
disrupted beam cleanly enough to allow extraction
line diagnostics
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Neutrons from the Beam Dump
Geometric fall off of neutron flux passing 1 mrad
aperture parent distribution for
next slide
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Dump-produced Neutron flux at z0 as a function
of radius

• 1.2E10 neutrons hit the beampipe within /-5cm at
  rgt1.0 cm
• 30 scatter into VXD
• Divide by area of VXD L1 to get quoted hit
  density 0.25E9/cm2/y
• Fall off for rgt1.0 cm due to limiting aperture of
  EXTRACTION LINE QUAD DOUBLET (currently 10-11 mm
  from L6-10.8 m from the IP SR concerns MAY
  require larger aperture)
• Fall off as r -gt 0cm comes from reduced solid
  angle view of the dump
• As r is reduced need to integrate more of this
  curve.

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Integrated Dump Neutron Flux vs. Radius

• As VXD inner radius is reduced by x2
• Flux from dump up x10
• Hit density up by x40 from 0.25E9 to 10E9
• dump becomes dominant source of neutron hits
• All numbers are from OLD simulation and may
  change if extraction line aperture increases

27
Synchrotron Radiation at SLC/SLD

• SR from triplet WOULD have directly hit beam-pipe
  and VXD
• Conical masks (M2) were installed to shadow the
  beam pipe inner radius and geometry set so that
  photons needed a minimum of TWO bounces to hit a
  detector
• Small cylindrical masks (M4) were placed within
  the beam pipe close to the IP to close a small
  one bounce window. These were a large source
  of charged DC hits (presumably due to off energy
  beam particles which scraped upstream, hit, and
  made 6 GeV pions)
• Quantitative measurements of background rates
  could be fit by a flat halo model where it was
  assumed that between 0.1 and 1 (in the early
  days) of the beam filled the phase space allowed
  by the collimator setting.

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NLC SR Design Criteria

• No direct SR hits ANYWHERE. Do NOT hit
• inner bore of QD0
• Conical M1 mask
• Be Ring Mask protecting VXD-L1
• Beam Pipe at VXD
• Extraction line beam pipe or magnet aperture

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Sources of Beam Halo

• Calculable contributions to the expected halo are
  SMALL. Per 1012 particles (1 train), we expect
• DR/RTL/Compressor ?
• Main Linac Wakefields lt107 (Tor)
• Captured Dark Current 104 (Brinkmann)
• Multipoles in main linac ?
• Linac coulomb/compton 104 (Tor)
• Linac mistuning ?
• BDS coulomb/compton 103 (ZDR)
• SLC experience 0.1 of beam in halo
• often thought to have come from DR/RTL but Panta
  says all RTL scans show Gaussian beam. Panta
  feels SLC problems arose from non-linear nature
  of the FF itself, and will GO AWAY with the new
  FF.
• Expect some improvement from pre-linac
  collimation and new FF
• Present design assumes 109 halo particles per
  train
• current (pre-Panta) thinking should try to
  collimate at 10-6 level (transmitted halo halo
  generated by edge scattering of the collimators)

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Basic Accelerator PhysicsAngular Divergence

• Emittance is as good as designers can deliver
• Beta function is set to give desired spot size
  and therefore luminosity
• Current NLC parameters sx 28 mrad, sy 40
  mrad at 500 GeV

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1 TeV Ray Plots
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SR Fan from 280 mrad Beam Particle
QF1
Photons from QF1 are the problem
Magnet apertures on input side very generous
QD0
Extraction Line aperture filled
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IP Close-Ups of SR Fans
280 mrad
200 mrad
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Stans SR Results
Assumes 1E-3 halo Eg 3-4 MeV Sextupoles not yet
included Effective vs. Ideal accounts for 1 beam
energy spread and planar collimation
Flare
Min. radius
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Collimator Phase Space

• How tightly you can collimate is a very slippery
  question
• Limiting apertures at the IP
• Minimum collimation apertures in the collimation
  section
• Lost particle heating
• Image current heating
• Wakefields
• Maximum number of muons generated (for a given
  halo)
• Radiation in the collimation section (for a given
  halo)

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Limits to Collimator Phase Space

• Available aperture is reduced by effects not yet
  considered in detail
• New FF has large angular dispersion (x/(DE/E))
  h 5.9 mrad at IP
• Allow for 1 energy spread at the IP 59 mrad
• Reduce aperture by ?2 to account for rectangular
  collimation in x, x, y, y
• 200 mrad in x is effectively 200-59/28/1.4
  3.6sigma
• At this level, the GAUSSIAN part of the beam
  distribution will start to contribute, so it is
  hard to imagine collimating any tighter than this

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Conclusion

• A VXD beam pipe of about 10mm is probably as
  aggressive as you want to get, especially before
  we understand the beam halo, energy spread, and
  backgrounds in more detail